10,902 research outputs found
Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions
The determination of the magnetic filed vector in solar filaments is possible
by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral
lines like those of the He I multiplet at 10830 A. We study the vector magnetic
field of an active region filament (NOAA 12087). Spectropolarimetric data of
this active region was acquired with the GRIS instrument at the GREGOR
telescope and studied simultaneously in the chromosphere with the He I 10830 A
multiplet and in the photosphere with the Si I 10827 A line. As it is usual
from previous studies, only a single component model is used to infer the
magnetic properties of the filament. The results are put into a solar context
with the help of the Solar Dynamic Observatory images. Some results clearly
point out that a more complex inversion had to be done. Firstly, the Stokes
map of He I does not show any clear signature of the presence of the filament.
Secondly, the local azimuth map follows the same pattern than Stokes as if
the polarity of Stokes were conditioning the inference to very different
magnetic field even with similar linear polarization signals. This indication
suggests that the Stokes could be dominated by the below magnetic field
coming from the active region, and not, from the filament itself. Those and
more evidences will be analyzed in depth and a more complex inversion will be
attempted in the second part of this series.Comment: 18 pages, 19 figures, accepted for publication in A&
Hypoxic Cell Waves around Necrotic Cores in Glioblastoma: A Biomathematical Model and its Therapeutic Implications
Glioblastoma is a rapidly evolving high-grade astrocytoma that is
distinguished pathologically from lower grade gliomas by the presence of
necrosis and microvascular hiperplasia. Necrotic areas are typically surrounded
by hypercellular regions known as "pseudopalisades" originated by local tumor
vessel occlusions that induce collective cellular migration events. This leads
to the formation of waves of tumor cells actively migrating away from central
hypoxia. We present a mathematical model that incorporates the interplay among
two tumor cell phenotypes, a necrotic core and the oxygen distribution. Our
simulations reveal the formation of a traveling wave of tumor cells that
reproduces the observed histologic patterns of pseudopalisades. Additional
simulations of the model equations show that preventing the collapse of tumor
microvessels leads to slower glioma invasion, a fact that might be exploited
for therapeutic purposes.Comment: 29 pages, 9 figure
A search for magnetic fields on central stars in planetary nebulae
One of the possible mechanisms responsible for the panoply of shapes in
planetary nebulae is the presence of magnetic fields that drive the ejection of
ionized material during the proto-planetary nebula phase. Therefore, detecting
magnetic fields in such objects is of key importance for understanding their
dynamics. Still, magnetic fields have not been detected using polarimetry in
the central stars of planetary nebulae. Circularly polarized light spectra have
been obtained with the Focal Reducer and Low Dispersion Spectrograph at the
Very Large Telescope of the European Southern Observatory and the Intermediate
dispersion Spectrograph and Imaging System at the William Herschel Telescope.
Nineteen planetary nebulae spanning very different morphology and evolutionary
stages have been selected. Most of central stars have been observed at
different rotation phases to point out evidence of magnetic variability. In
this paper, we present the result of two observational campaigns aimed to
detect and measure the magnetic field in the central stars of planetary nebulae
on the basis of low resolution spectropolarimetry. In the limit of the adopted
method, we can state that large scale fields of kG order are not hosted on the
central star of planetary nebulae.Comment: Paper accepted to be published in Astronomy and Astrophysics on
20/01/201
From circular paths to elliptic orbits: A geometric approach to Kepler's motion
The hodograph, i.e. the path traced by a body in velocity space, was
introduced by Hamilton in 1846 as an alternative for studying certain dynamical
problems. The hodograph of the Kepler problem was then investigated and shown
to be a circle, it was next used to investigate some other properties of the
motion. We here propose a new method for tracing the hodograph and the
corresponding configuration space orbit in Kepler's problem starting from the
initial conditions given and trying to use no more than the methods of
synthetic geometry in a sort of Newtonian approach. All of our geometric
constructions require straight edge and compass only.Comment: 9 pages, 4 figure
CMB anisotropy: deviations from Gaussianity due to non-linear gravity
Non-linear evolution of cosmological energy density fluctuations triggers
deviations from Gaussianity in the temperature distribution of the cosmic
microwave background. A method to estimate these deviations is proposed. N-body
simulations -- in a CDM cosmology -- are used to simulate the strongly
non-linear evolution of cosmological structures. It is proved that these
simulations can be combined with the potential approximation to calculate the
statistical moments of the CMB anisotropies produced by non-linear gravity.
Some of these moments are computed and the resulting values are different from
those corresponding to Gaussianity.Comment: 6 latex pages with mn.sty, 3 eps figures. Accepted in MNRA
Hierarchical analysis of the quiet Sun magnetism
Standard statistical analysis of the magnetic properties of the quiet Sun
rely on simple histograms of quantities inferred from maximum-likelihood
estimations. Because of the inherent degeneracies, either intrinsic or induced
by the noise, this approach is not optimal and can lead to highly biased
results. We carry out a meta-analysis of the magnetism of the quiet Sun from
Hinode observations using a hierarchical probabilistic method. This model
allows us to infer the statistical properties of the magnetic field vector over
the observed field-of-view consistently taking into account the uncertainties
in each pixel due to noise and degeneracies. Our results point out that the
magnetic fields are very weak, below 275 G with 95% credibility, with a slight
preference for horizontal fields, although the distribution is not far from a
quasi-isotropic distribution.Comment: 9 pages, 4 figures, accepted for publication in A&
The CIB-lensing bispectrum: impact on primordial non-Gaussianity and detectability for the Planck mission
We characterize the cosmic infrared background (CIB)-lensing bispectrum which is one of the contributions to the three-point functions of cosmic microwave background (CMB) maps in harmonic space. We show that the CIB-lensing bispectrum has a considerable strength and that it can be detected with high significance in the Planck high-frequency maps. We also present forecasts of the contamination on different shapes of the primordial non-Gaussianity fnl parameter produced by the CIB-lensing bispectrum and by the extragalactic point sources bispectrum in the Planck high-resolution CMB anisotropy maps. The local, equilateral and orthogonal shapes are considered for âraw' single-frequency (i.e. without applying any component separation technique) and foreground-reduced Planck temperature maps. The CIB-lensing correlation seems to mainly affect orthogonal shapes of the bispectrum - with and â88 for the 143 and 217 GHz bands, respectively - while point sources mostly impact equilateral shapes, with and 60 at 100, 143 and 217âGHz. However, the results indicate that these contaminants do not induce any relevant bias on Planck fnl estimates when foreground-reduced maps are considered: using SEVEM for the component separation, we obtain due to the CIB-lensing and due to point sources, corresponding to 0.3Ï and 0.45Ï in terms of the Planck 2013 fnl uncertainty. The component separation technique is, in fact, able to partially clean the extragalactic source contamination and the bias is reduced for all the shapes. We have further developed single- and multiple-frequency estimators based on the Komatsu, Spergel & Wandelt formalism that can be implemented to efficiently detect this signa
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